Equipment for measuring phase-sensitive impedance or admittance (collectively, vector parameters), or phase insensitive resistance or conductance (collectively, scalar parameters) on batteries, particularly, but not limited to, lead-acid batteries, is widely available. Technical expertise is often needed to interpret the raw data outputted by such instruments, and to understand the implications of the data for battery maintenance and replacement.
It is presently not possible to accurately and consistently correlate impedance parameters with a condition of a battery, such as state of charge (SOC), or retained capacity (RC), cold cranking amps (CCA), or with post-mortem assessments of failure modes such as sulfation, dryout, or electrolyte decomposition (all broadly categorized as State of Health (SOH) characteristics), without using some sort of reference value as a baseline with which to compare parameter readings obtained. The accuracy of determinations of a SOC or SOH condition is dependent on the accuracy of the baseline used. However, there is at present no entirely satisfactory method available for obtaining baseline values for measured vector and scalar parameters of batteries.
This same fundamental need for a baseline reference applies whether the user is attempting to determine a condition of an individual battery, of a battery in a network, or of a network of batteries as a whole. It applies when the user is attempting to screen a population of non-networked batteries for purposes such as deciding whether to include the individual batteries in a network. It also applies whether the user is utilizing a snapshot or a continuous approach to testing.
Some battery manufacturers, and some battery test equipment manufacturers, publish reference lists of typical conductance or other scalar parameters for specific battery models in an effort to address this issue. The assumption is made that a reference scalar parameter value obtained from a generic, new battery will provide a valid baseline. But, there are several problems with reference lists derived under pristine conditions from battery samples that are not specific to the actual device during actual use in the field.
First, scalar parameter measurements tend to be instrument-specific, as there is a wide variety of techniques and frequencies used by the different manufacturers, and it is well known in the industry that the results from different instruments, and even different instruments of the same make and model, are far from identical.
Second, although a typical scalar reference parameter value derived in this way represents the mean for a large number of batteries, values for individual batteries can deviate widely from the mean. In fact the range of values found among nominally identical batteries from a single manufacturer often exceeds the range in values expected for a single healthy battery over its lifespan. Furthermore, average parameter values can change substantially with even minor manufacturing changes.
Third, the temperature dependence of reference values is typically not considered. Vector and scalar measurements of electrochemical systems vary markedly with temperature. This variation is typically not taken into account with published reference values.
Fourth, reference values collected under ideal conditions may bear little similarity to the true reference values for individual batteries in their operating environment. At a minimum, these reference values are universally collected on batteries at open circuit potential, while many field measurements are done on batteries being actively charged, which changes the vector and scalar measurement parameters of the battery. Furthermore, it is well known in the industry that most types of battery, and particularly lead acid batteries, undergo the final stages of their formation processes after they enter use. The vector and scalar parameters of the battery will change with these final bedding in stages and any accurate reference value must take account of these changes.
Fifth, published reference scalar parameter values typically refer to a single frequency point, or to a single DC measurement value. Multi-frequency battery testing equipment is now emerging, with a potentially infinite number of frequencies at which both scalar and vector parameters can be measured. These parameters may vary markedly with frequency. Thus a single scalar reference value offers minimal or no utility to these instruments.
Therefore, what is needed are techniques for automatically determining baselines for battery testing of batteries under actual operating conditions and specific to the test instrument model in use, and the vector and scalar parameters, or any other measurement parameters, it is designed to measure.